Austenitic stainless steel, especially austenitic stainless steel, has been widely employed for its superior corrosion resistance property and excellent processability. However, the above austenitic stainless steel and the like do not have quenching hardenability and also are not so superior in processing hardenability. Therefore, they are not suitable for the use for parts demanding high wear resistance.
Thus, austenitic metal represented by austenitic stainless steel has superior corrosion resistance property and excellent processability, however, the austenitic metal also has a drawback of being easily damaged due to low hardness, which causes a big problem. Generally, besides the above quenching, there are methods such as 1 carburization, 2 nitriding and the like for improving hardness. The carburization is a method comprising steps of heating low carbon steel or low alloy steel at a temperature not less than A1 transformation temperature (approximate 720.degree. C.), maintaining it as an austenite phase, spreading "C" to be penetrated into the surface of the above steel under RX gas or gas mixture containing CO so as to be hardened. Carburizing is conducted usually at a temperature not less than A1 transformation temperature (approximate 720.degree. C.) since solubility of "C" to a ferrite phase is extremely low at a temperature not more than 700.degree. C. in case of steel other than austenitic metal.
By the way, it is said that anti-corrosion property to the above austenitic metal represented by austenitic stainless steel emerges due to a passive coat layer produced on the surface, which includes Cr.sub.2 O.sub.3. This passive coat layer is strong even in the temperature range of 300.degree. to 700.degree. C. and prevents not only penetration of corrosive substances but also penetration of nitrogen atoms and carbon atoms and the like which are employed for nitriding and carburizing.
As the above 1 of carburizing the austenitic metal wherein such a passive coat layer is formed, there is a method that the austenitic metal is heated over 700.degree. C. to destroy or weaken the above passive coat layer and then carbon atoms are penetrated thereon. Carburizing is impossible and actually has not yet been put into practical use because the passive coat layer exists at a temperature not more than 700.degree. C., greatly lower than A1 transformation temperature of steel.
However, if the austenitic metal is heated over 700.degree. C. as mentioned above, the passive coat layer is removed or weakened but also the overall strength is deteriorated because the inner part (the core) of the austenitic metal itself softens, wherein minimum strength for mechanical parts cannot be retained. Therefore, carburizing for austenitic metal has scarcely conducted industrially heretofore.
On the other hand, as the above 2 of nitriding austenitic metal for improving the hardness, there are mainly the following three methods heretofore.
A first method is salt bath nitriding employing NaCNO, KCNO or the like, which weakens the passive coat layer on the surface of the above austenitic metal by setting up a temperature at 500.degree. to 600.degree. C., so that nitrogen atoms are penetrated therein.
In a second method, firstly the above passive coat layer on the surface of the above austenitic metal is destroyed and removed by sputtering, and then the austenitic metal is nitrided with N.sub.2 gas, NH.sub.3 gas or the like.
According to these two methods, the passive coat layer is weakened or removed, so that nitrogen atoms penetrate into the inside of the austenitic metal to some extent. However, there is a drawback that anti-corrosion property, the characteristic inherent in austenitic metal, greatly deteriorates because chrome concentration on the surface lowers.
Besides, there is another problem that the surface roughness becomes worse by nitriding and also dimension accuracy deteriorates because the austenitic metal itself swells by introduction of nitrogen atoms. In addition, there is still another problem that the austenitic metal itself is magnetized by the above nitriding.
The third method, which we developed and made a patent application to Japan Patent Office (application number JP1-177660 and JP1-333424), is that the surface of the austenitic stainless steel is heated under fluoride-containing gas atmosphere such as NF.sub.3 prior to the above nitriding. In this method, the passive coat layer including CrO.sub.3, formed on the surface of the austenitic stainless steel, is converted into a fluorinated layer by the above previous treatment, and then nitriding treatment is conducted under normal condition. Thereby, nitrogen atoms in nitrogen gas penetrate into the austenitic stainless steel through its surface to a specific extent uniformly so as to form a deep uniform nitride layer and at the same time the above fluoride layer is removed by contacting with moisture or hydrogen and the like in nitriding gas. Compared with the above 1 and 2, our method enables more excellent nitriding, resulting in austenitic stainless steel with preferable hardness. However, even in our method, the same problems may be caused depending on circumstances; anti-corrosion property may deteriorate, the surface roughness may become worse, nitride articles may swell or be magnetized, which requires improvement.